I like the combo on my Emerson CQC8 just because the blade is long enough to use only the plain edge if I want, and use the serrated edge when needed. It doesn't take much of a pull cut with a serrated edge to get the job done most of the time for me, anyway. YMMV.
Was walking the dog one evening, when I decided to cut back a 1/2" thick branch that hung way out over the sidewalk and got in our way all the time. Pulled out my shaving-sharp plain edge Boker Ginger Fighter and thought "this'll be a snap!" That thing is sharp as hell.
Started sawing, and sawing, and sawing. Was hugely disappointed.
Pulled the 8 out of the other pocket (I always carry two knives) and with one pull, in a different spot, with the serrated portion of that blade that branch was a goner.
So IMO, a combo edge can be useful if the blade itself is long enough to have a true choice.
I cut a seatbelt off a woman in a wreck once with that same knife with the serration and it took literally 1 second and she was free. I purposely moved the blade down to the serrated part before engaging the belt and it worked like a charm.
I wanted the blade itself near her as little amount of time as possible, as it was not a blunt-ended "rescue" type blade.
And my pretty-much fully-serrated Spyderco Native black cuts like a champ. I often carry one fully-serrated blade in one pocket and a PE in the other. They each have their better uses.